Nice shot. But I think I would rather have the 500 than the 300 for this kind of shot! LOL! CPS loaned me their 500 for a couple of weeks and I found it a challenge trying to find the subject when it was in flight. for stationary objects it was great, especially on the wimberly. As you state the MTF on the new models offer theoretical advantages but I'm curious to how they relate to real world usefulness. Reach is obviously an advantage shooting most any wildlife...you often dont have the opportunity to get closer...even with bears.
How important to you is f2.8 vs F4 or F5.6 in the scheme of things?All depends on how close you expect to get to your subject. Here's a Black Bear sow shot with the 5D Mark II and the old non IS 300mm f2.8. Saying that I would go for the 500. I've had my 500 since November of 2001 and it is my most used lens. The new 500 Mark II should be here next week and I can't wait. The new 500 to me looks like the sweet spot, especially if you can believe the MTF curves that Canon publishes.
If I had the 500 on I would have had to put the 25mm extension tube on too to get her in focus.